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Whales occupy an outsized place in our imagination. They are mammals, like us, but grand and mysterious, from their enormous size to their social lives to the great depths they swim in.

For all our fascination with whales, there’s also a surprising amount we don’t know about them—including basic information like how old individual whales are, or how long they’re capable of living. Marine researchers have long sought an accurate technique for “aging” whales, because knowing how old whales are is key to answering many other questions about their lives. But such a technique has been elusive—until now.

Last month researchers at the Provincetown Center for Coastal Studies, in conjunction with a team of biologists from Australia, announced they had found a new, and, they believe, much more accurate way to determine the ages of humpback whales. In a paper published in the journal Molecular Ecology Resources, they describe the method, called a humpback epigenetic age assay, which involves scraping skin from live humpback whales and analyzing the molecules that collect around their DNA over time.

“This is the most accurate and widely applicable aging method,” said Jooke Robbins, a population biologist at the Center for Coastal Studies and coauthor of the study. “It’s mind boggling to me the wealth this method will bring to whale research.”

Determining the exact age of any organism is difficult if you don’t know when it was born, and whales present particular challenges. They are clearly long-lived, and are often older than the researchers studying them; baleen whales such as humpbacks have no teeth, which is how age is often measured in mammals; it’s hard to get any kind of accurate measurement on them in the ocean; and there are no reliable visual differences that can be used to distinguish a 5-year-old whale from a 50-year-old whale.

“We’ve had cases,” Robbins said, “where individual [humpbacks] known to be mature animals were thought to be yearlings based on visual perception.”

Earlier aging techniques ranged from measuring the concentration of fatty acids in living whales to taking earplug samples from dead whales after they’d washed ashore. All of these methods are easily skewed by variations in individual whale biology, the ocean environment, or our own lack of understanding of the biological processes we’re measuring. The earplug technique, for example, depends on knowing how many layers of earwax a whale produces each year. When biologists revised their earwax production estimates in 2010, said Robbins, the age of the oldest known humpback jumped from 48 years to 95 years.

The new assessment method is more technical. First, Robbins and her research team used crossbows to fire darts at humpbacks in the Gulf of Maine. Each dart collected a skin sample about a quarter-inch deep before bouncing into the water, where the biologists retrieved it. The Center for Coastal Studies had been tracking these particular whales since birth, based on the unique markings on the undersides of their flukes, so they already knew how old they were. That made this population a unique test case—if the researchers could produce age estimates from skin samples that matched observed ages, they’d know their new method was accurate.

Back in the lab, they analyzed the number of methyl groups—combinations of one carbon atom and three hydrogen atoms that occur commonly in nature—that had adhered around a few key spots in each whale’s DNA. The number of methyl groups around some specific genes changes at a steady rate, and has been shown in humans and mice to control the genetic basis of aging—they help to explain why, for instance, our hair goes gray over time. Based on these numbers, the researchers estimated the age of each sampled whale and compared those estimates to the whales’ known ages. On average, their estimates were accurate to within 2.9 years.

“This is the first good way to be able to take a biopsy and [calculate age],” said Cheryl Rosa, deputy director of the US Arctic Research Commission and a wildlife veterinarian who has worked on previous whale aging studies. She expressed “guarded optimism” that this new method will work equally well on other whales that have been less closely observed than this particular population of humpbacks.

Not everyone is convinced it’s a surefire approach. The researchers only had observation data on humpbacks going back 35 years, which means they couldn’t test their method to see if it works in humpbacks older than that. In an e-mail, Jeffrey Bada, a marine chemist at the Scripps Institution of Oceanography, expressed skepticism that methyl groups collect at a consistent enough rate to be used to calculate age, especially in older whales. But Simon Jarman, a molecular biologist and coauthor of the new paper, replied that around certain genes, as seen in mice and humans, the rate is consistent. The researchers focused on particular gene sites where the aging signal is strong, and say there’s every reason to believe this process would be consistent in whales, too.

Age is such a simple measurement—and so universally known among human beings—that it’s easy to take for granted how essential it is to all kinds of research questions. Robbins said, “From a biologist’s point of view, it’s almost magical to know the age.”

Knowing whales’ ages will allow biologists to distinguish parent whales from their offspring, and to measure the balance of older and younger whales in a population—which in turn helps biologists determine whether a population is growing or shrinking.

Age measurements are also a prerequisite for answering many health-related questions, like whether female whales experience menopause and how aging and environmental pollutants contribute to health outcomes. To put this last point in human terms, we’d think about the causes of a heart attack very differently depending on whether the patient was 30 or 80 years old.

There’s also a more intangible value to being able to calculate how old whales are. Age and longevity shape the way we think about other people and other things—we revere the ancient redwoods, marvel at newborn babies, find poignancy in the fleeting lives of flies. Knowing how old whales are is a big part of knowing them at all. When a bowhead whale caught in 2007 turned out to have a harpoon tip embedded in its skin that dated from the 1890s, it was evidence that some of these animals might be older than any human on earth.

“One of the first questions you get from people is how long do whales live,” Robbins said. “And we’ll probably know that by next year.”